1. Field of the Invention
This invention relates to geared, automatic power transmissions for motor vehicles. More particularly, it pertains to the kinematic arrangement for such transmissions and a driveline that incorporates them.
2. Description of the Prior Art
A transfer case contains a drive mechanism for connecting an engine to the vehicle's wheels, a primary wheel set that is connected continually to the engine, and a secondary wheel set that is connected selectively to the engine through the transfer case. A two-speed transfer case produces both a high range and a low range of speed ratios. Each of the speed ratios produced by the transmission is operative in each of the drive ranges produced by the transfer case; therefore, the transmission-transfer case can produce in combination a multiple of the transmission gear ratios. In this way the transfer case provides greater functional flexibility to the vehicle operator, who has a wider range of speed ratios to select depending on road conditions, towing load, vehicle speed, and other drive conditions.
Most operators of 4×4 vehicles, those vehicles having a driveline that transmits engine power to front and rear wheels, rarely operate the driveline in the Low Range, the range in which the lowest speed ratios are produced. A principal reason for this is the need to stop the vehicle before changing the drive state of a transfer case clutch that must be engaged in order to produce the Low Range. The wider range provided by new six speed automatic transmissions will likely cause still less frequent use of the Low Range, and decrease the need for two-speed transfer cases to meet the requirements of vehicle operators.
Certain vehicle lines require an overall transmission/transfer case gear ratio of approximately 12:1 in combination with relatively low engine speed, i.e. 2–3 mph @1000 rpm. Other vehicle lines have much less much less stringent requirements, i.e., lower gear ratios at low engine speed.
Automatic shifting under full torque from the low speed range to high speed range is a goal in driveline design, but the speed ratio step, from the low range to high range, in current transfer cases is too large for good shift quality and would need to be reduced to approximately 1.8 or less. However, performance degradation due to the resulting lower gear ratios would be undesirable. The use of multi-plate clutches in the transfer case to shift from low to high, coupled with six-speed automatic transmissions would result in added parasitic drag and lower fuel economy. In certain applications where a 12:1 overall transmission/transfer case speed ratio is required, a three-speed transfer case would be necessary with currently available six-speed automatic transmissions. But this would compound the losses.